Nucleolus organizer regions in translocations involving acrocentric chromosomes

Twenty-seven Robertsonian translocations and five translocations involving one acrocentric and one nonacrocentric chromosome were examined with the aid of three staining methods. The chromosomes were identified by Q-banding, and the slides were then destained and restained with a silver impregnation method for nucleolus organizing regions (NORs). Replicate slides were C-banded. The centromeric region of one familial (15q21q) translocation chromosome had a large silver block and two widely separated blocks of C chromatin. Also, the centromeric region of the small translocation chromosomes in a de novo (13;15) translocation was heavily stained with silver. This was the only case where both reciprocal products of a Robertsonian translocation were found. In the other 25 Robertsonian translocations examined, no NOR staining regions could be identified, although five chromosomes each had two centromeric heterochromatic blocks. In the (1;13), the (7;13), and the (12;21) translocations, one of the two chromosomes involved in the translocation retained the NOR region. This was not true for two other cases, one a t(6;15) and the other a t(9;15). An evaluation of the number of NOR regions revealed that in the cells of balanced carriers there was a maximum of six to eight NOR sites, whereas in the unbalanced state the maximum number was six to nine. In the t(13ql4q) the silver-stained sites on other acrocentric chromosomes were larger. This was rarely found to be the case in the other types of Robertsonian translocations.